Diesel engines, and particularly small Diesel engines which are used for example in automotive vehicles, may have one or more glow plugs which are connected in an electrical circuit. The glow plugs extend into the cylinder of the engine, to provide for preheating of the cylinder wall and air being compressed in the cylinder as the engine is being started. The glow plugs are energized shortly before starting of the engine. Just after starting, the glow plugs can continue to be energized, however with reduced power, depending on the temperature then pertaining in the interior of the cylinder, so that continued combustion in the respective cylinders can be sustained upon injection of fuel into the compressed air.
It is customary to include a control indicator, for example a control lamp, in the circuit of the glow plugs. Some arrangements also include a monitoring system or monitoring circuit to check and supervise the operation of the glow plugs.
The glow plug circuit, or overall glow plug system, can be controlled from a central motor control unit. Such central motor control units may be electronic units which store in a suitable memory various control commands, which are supplied to a decoding circuit which, in turn, controls the respective components of the glow plug system.
Decoding circuits, which decode commands to control sequential operation of glow plugs--for example with different energy levels in dependence on engine operation or temperature--are known. Decoding circuits have to be matched to the specific application after they have been built. If, after the decoding circuit has been built, it becomes desirable to change the engine operating characteristics, for example by changing starting speed or the like, it is not readily possible to change the decoding circuit to match the changed engine characteristics. The decoding circuits are usually part of the interface between the engine control unit and the glow plug circuit. Monitoring of operation of the glow plugs is usually commanded by the engine control unit; the interface, as such, in the past has not carried out any monitoring functions, that is, could not carry out on its own a checking or monitoring subroutine.